U.S. Pat. No. 4,135,124 in the name of one of us, Robert Buck, discloses a monitoring system of the type here considered which includes logic circuitry for selectively presetting that system to respond to the occurrence of an ambient condition being sensed, i.e. to a change in the value of a binary signal resulting therefrom, with either an intensification or a reduction of the load current. This is accomplished with the aid of an Exclusive-OR (XOR) gate, of the regular or the negated type, which has a first input provided with switchover means for selectively applying two different voltages to the latter input, either manually or automatically. Depending on the logical value of the applied voltage, the XOR gate transmits either the unmodified binary signal or its logical complement to a control input of an electronic switch such as a thyristor. Thus, the XOR gate acts a selective signal inverter whereby, for example, a mechanical relay controlled by the electronic switch can either open or close a load circuit upon the occurrence of the event being monitored.
It is also known, e.g. from German printed specification Nos. 29 31 879 and 29 31 880 respectively published Feb. 19, 1981 and Oct. 16, 1980, to provide two thyristors controlled by a common detector which includes an oscillator whose output amplitude is modified by the occurrence of the monitored event. The two thyristors are operated in an antivalent mode, one of them being fired at high amplitude and the other being fired at low amplitude. Either thyristor, when conducting, intensifies the energization of a respective load; alternatively, a single load may be connected in circuit with one or the other thyristor so that its energization is either intensified or reduced when the monitored event occurs. The detector, energized by the same source, is not completely short-circuited upon the firing of either thyristor since, as explained in the German description, the conductive thyristor is cut off upon each zero-crossing of the alternating supply voltage and does not fire again until the absolute magnitude of the voltage reaches its conduction threshold.